Insulation is a necessary component of any cryogenic system. Commercial cryogenic equipment, such as a cold box used to liquefy and separate air to produce product oxygen and nitrogen, is usually insulated with rock wool, perlite or similar inorganic fiber or powder product. Rock wool is a mineral wool made by blowing a jet of steam through molten rock or through slag. Often these insulation materials have excess sodium, calcium or other alkalai elements which produce a high pH solution when water is present.
Aluminum alloys, preferred materials of construction for cryogenic equipment, are strong, lightweight, readily welded and exhibit minimal brittle behavior at low temperatures. They are also inexpensive when compared with other metals possessing attractive cryogenic properties. However, unlike most other metals, aluminum alloys suffer catastrophic corrosion in high pH solutions because of the amphoteric nature of aluminum.
Over time, moisture enters the equipment and usually freezes, forming ice balls within the insulation. This poses no problem until a defrost cycle is undertaken. Defrosting is necessary to make changes or repairs on equipment. During defrosting, the liquid moisture will usually migrate to the aluminum surfaces and cause extensive corrosion damage to the equipment before starting up the equipment, especially if the equipment is being taken out of service for an extended period. Equipment repair is costly.
In order to minimize problems from corrosion due to defrosting, the cold box of the cryogenic system may be pressurized with dry nitrogen and the box kept sealed. During defrosting, dry nitrogen is passed through the box to evaporate the moisture before it has a chance to migrate to the surfaces. Purging of the rock wool with dry nitrogen is only partially effective. Alternatively, the rock wool may be removed from the cold box during the defrosting to minimize damage, but this is a very expensive process which is both lengthy and difficult. In spite of all of these measures, corrosion still occurs from ice balls formed in the insulation.
Another alternative is to use rock wool which is acid treated or has a lower pH when mixed with water. This approach is very expensive and limited by the availability of such products. Also, it is not useful in systems where untreated rock wool is already in place.